It is well known in the prior art that it is desirable to provide a vibration absorber that can absorb vibrations in a vibrating member.
One prior art technique for absorbing vibrations comprises a tuned vibration absorber. Such a passive absorber is illustrated in FIG. 1. As illustrated in this diagram, a vibrating member in which the vibrations are to be reduced by absorption comprises a base 1. An inertial mass 2 is stiffly mounted to the base 1 by a stiffness element which is illustrated as a spring member 3. As is well known in the art, the mass and the stiffness of the spring can be selected or tuned to a particular frequency to provide absorption at the frequency. At the particular chosen frequency resonance occurs by movement of the inertial mass 2 thus causing an absorption of the vibrations in the base 1. FIG. 2 is a graph illustrating the amplitude of vibrations in the mass 2 when the mass is mounted to the base 1 in an undamped manner.
It can be seen in FIG. 2 that although the undamped vibration absorber provides for good absorption at the resonance frequency, the frequency range is limited. Thus, it is known to provide a damper 4 to damp the vibrations between the mass 2 and the base 1. The damping provides for a reduction in the peak height of the resonance and a broadening of the peak. This is illustrated in FIG. 2.
The well-known passive tuned vibration absorber is limited in that it has a narrow frequency response. If the absorption of more than one frequency is required, typically more than one tuned vibration absorber is required. Also, the resonance frequency of the tuned vibration absorber is fixed by the effective mass of the inertial mass 2 and the spring stiffness of the spring member 3.
A known active vibration absorber is illustrated in FIG. 3. An inertial mass 11 is mounted to a base 10 via a spring arrangement 12. An actuator 13 is provided to provide a force between the mass 11 and the base 10. A sensor 14 senses vibrations in the base 10 and provides for an error signal which is input to a controller 15 having a gain G to generate a control signal for the actuator 13. Controller 15 comprises an adaptive controller which controls the actuator 13 in order to reduce the vibrations sensed by the sensor 14. As the controller 15 achieves control, i.e. vibrations are absorbed in the base 10, the error signal 14 provided for feedback control to the controller 15 reduce in amplitude and will tend towards zero. It is thus necessary for the controller 15 to have a very high feedback gain in order to provide for control. This provides a problem with stability. Further, although some passive damping can be provided between the mass 11 and the base 10, this is not actively controlled.